1. Field of the Invention
The present invention generally relates to a semiconductor device, and more specifically relates to a FeRAM (ferroelectric random-access memory) which stores information in a ferroelectric capacitor containing a ferroelectric film. Moreover, the present invention relates to a fixture for measurement of electrical property of a ferroelectric film capacitor within a semiconductor device.
2. Description of the Related Art
In recent years, a flash memory (electrically-rewritable non-volatile memory) which supplies the information in the form of charge to the floating-gate electrode has been often used as the auxiliary memory of a personal computer or the like. On the other hand, there has been proposed a FeRAM (ferroelectric random-access memory) which stores information in a ferroelectric capacitor in the form of spontaneous polarization of the ferroelectric capacitor, instead of the floating gate electrode as in the flash memory.
In the FeRAM, the ferroelectric materials used in the ferroelectric capacitor include PZT (Pb(Zr, Ti)03), PLZT (Pb(Zr, La, Ti)03), each having the perovskite crystal structure, and bismuth layered ferroelectric substance oxide SBT (SrBi2Ta2O9) having the layered perovskite crystal structure. In the FeRAM, the spontaneous polarization of the capacitor is controlled by the electric field applied, and it has the advantages of high writing speed and low power dissipation. The FeRAM is appropriate for use in a large-scale integrated circuit.
FIG. 1 shows the composition of a memory cell in a conventional FeRAM. As shown in FIG. 1, the memory cell is arranged in the 2T2C composition, and two transfer transistors T1 and T2 and two capacitors C1 and C2 are used in order to store the 1-bit information. The memory cell carries out such complementary operation that the data “0” is stored in one of the capacitors and the data “1” is stored in the other capacitor.
Specifically, when writing information, the transfer transistors T1 and T2 are turned ON by the word line WL, and the data “0” or “1” is inputted to the bit line BL and the data “1” or “0” is inputted to the complementary bit line /BL, so that the complementary information is written in the capacitors C1 and C2 respectively. The complementary information is held in the capacitors as the direction of polarization of the ferroelectric materials of the capacitors.
Moreover, when reading the information, the transfer transistors T1 and T2 are turned ON by the word line WL, and the voltage equivalent to the voltage by the polarization of the capacitors C1 and C2 is applied to the bit line BL and the complementary bit line /BL, so that the sense amplifier 202 reads out the information held by the capacitors according to a difference between the voltage of the bit line BL and the voltage of the complementary bit line /BL.
It has been reported previously that during the integration of the ferroelectric film into a device, under certain processing conditions degradation of the electrical property of the ferroelectric film which is a dielectric substance of the ferroelectric capacitor in the FeRAM arises over time, and read operation sensing margin of the FeRAM capacitor is reduced.
FIG. 2 shows the electric field versus polarization characteristics of a ferroelectric material.
As shown in FIG. 2, the ferroelectric film has the hysteresis characteristics. For example, when the electric field is applied higher than 2.5 times the negative coercive force −Ec with the ferroelectric film initially in the state of remanent polarization +Pr, will be reversed. The polarization will be saturated when the applied electric field is −Es. which is approximately 2.5 times the Ec or higher. And when the applied electric field is released, it will be in the state of the negative remanent polarization −Pr.
The change of the coercive force Ec and the loss of the remanent polarization Pr are major problems among the degradation of the electrical properties of the ferroelectric film.
It is known that the phenomena originate due to the trapped charge at the interface between the lower electrode layer and the upper electrode layer which sandwiches the ferroelectric film, or the point defect such as oxygen vacancies of the ferroelectric film. Moreover, it is known that degradation of the above-mentioned electrical property will be accelerated by heating or irradiation of UV light.
In addition to the above causes, there is the problem that the hysteresis characteristic of the ferroelectric film will change with the stress applied to the ferroelectric film. For example, the hysteresis characteristic of the ferroelectric film is shifted to positive or negative direction of the applied voltage. In such a case, the coercive voltage Ec changes, and even if a normal writing voltage is applied, the polarization is not reversed by the writing operation.
The stresses applied to the ferroelectric film include the stress applied to the interface of the ferroelectric film by the upper electrode directly formed on the ferroelectric film, the stress by the interlayer dielectric film, the passivation film or the circuit pattern used for multi-layer interconnection structure, and the stress guided to the ferroelectric film with the stress applied to the whole substrate when the ferroelectric film is formed.
Moreover, a silicon substrate having a thickness of about 0.65 mm is usually used for manufacturing FeRAM. Before the semiconductor circuit of FeRAM is formed and packaged, the grinding of the backside of the substrate is carried out, so that the silicon substrate is thinned. When the silicon substrate is thinned, there is an expectation that the influence by the above-mentioned stress by the interlayer dielectric film or the like increases further.